1. Field of the Invention
This invention relates to a flow imaging cytometer for picking up the fluorescent images of cells.
2. Description of the Prior Art
A laser light source often is used as an excitation light source in flow cytometers for measuring fluorescence emitted from unstained cells or from cells that have been treated with a fluorescent stain. The reason for employing such a light source is that use of a laser makes it possible to narrow down the zone measured, as a result which the intensity of irradiation per unit area in the measurement zone can be increased to strengthen the intensity of the fluorescence obtained from the irradiated cells.
A problem which arises when using a laser is that the excitation wavelength is limited to a specific wavelength, as a consequence of which a limitation is placed upon the fluorescent stain solutions that can be used. In addition, a laser light source is large in size and high in cost.
A method adopted in an effort to solve these problems involves using an xenon lamp or the like as the excitation light source and selecting the wavelength of the excitation light by using an interference filter as a filter which selects the excitation light. However, since the irradiated zone cannot be narrowed down in the manner made possible by a laser, a high fluorescent intensity is not obtained. The problem involving the intensity of the excitation light is conspicuous in an apparatus for picking up a fluorescent image, as set forth in the specification of Japanese Patent Application Nos. 3-33151, 3-33189, 3-33137 and 3-33138. The following methods have been adopted in order to intensify fluorescence for the sake of obtaining the fluorescent images of cells:
1. An excitation light source having a high light-emission intensity is used. PA1 2. The excitation light is narrowed down to the smallest zone possible. PA1 3. Weak fluorescence is intensified by an image intensifier, which is a photomultiplier element.
If the intensity of the fluorescence emitted from a cell is low, however, applying excessive amplification by an image intensifier results in a lower S/N ratio due to photoelectric noise. The resulting drawback is a deterioration in image quality. This means that there is a limit upon the maximum mu-factor that can be used, and hence the intensity of the fluorescence necessary for picking up the fluorescent image of a cell is inadequate.